The present invention relates to a process for the production of polyurethane block foam using carbon dioxide as blowing agent.
In such processes, preferably liquid carbon dioxide is mixed under pressure with preferably one of the components, especially the polyol component, and then the component containing dissolved carbon dioxide is introduced into a mixing chamber, where it is mixed with the other component, typically the isocyanate component, and additional auxiliary substances. To produce a uniform foam, the pressure of the polyol/isocyanate mixture containing dissolved carbon dioxide has to be reduced suddenly to normal pressure so that a large number of bubble nuclei are formed in the mixture as uniformly as possible and grow into uniform foam bubbles. The resulting liquid foam is transferred to a conveyor belt, where it may continue to foam due to the presence of additional blowing agents, and also cures. A particularly suitable additional blowing agent is water, which forms carbon dioxide by reaction with the isocyanate.
In accordance with a number of proposals made by the Applicant (EP-A 767 728, EP-A 777 564 and EP-A 794 857), the expansion is effected by forcing the polyol/polyisocyanate mixture containing carbon dioxide through a series of several interconnected screens or perforated plates having a large number of apertures with diameters in the order of 100 xcexcm. Such perforated plates typically have several tens of thousands of drilled holes or through-holes. Because the screens or perforated plates are arranged in close proximity to one another, the expansion takes place within very short periods of time and each of the several tens of thousands of streams of polyol/polyisocyanate mixture passing through the perforated plate experiences high shear rates, generating a high and uniform bubble nucleus density of several hundred thousand bubble nuclei per cm3 in the mixture. The supersaturation of the mixture produced by pressure reduction results in the generation of a corresponding number of very uniform foam bubbles. The number of perforated plates to be used, the size of the through-holes and the total free cross-section of the perforated plates are chosen as a function of the foam quality to be produced (carbon dioxide content of the mixture, foam density, pressure before expansion). There is only a limited possibility of varying the foam quality while the operation is running, i.e. without changing the perforated plates. Furthermore, because of the small diameter of the screen holes, there is a risk that they may become clogged over prolonged operating periods by fine solid particles present in the polyol/isocyanate mixture.
The production of filled foams remains an unsolved problem. Even very finely divided fillers with typical particle sizes of 10 xcexcm block the screens due to the tendency of the filler particles to agglomerate.
The object of the present invention is to improve the bubble nucleation in the polyol/isocyanate mixture so that it is not necessary to use fine-pore screens, and also to allow the use of fillers in polyurethane foams produced with carbon dioxide as blowing agent.
One particular object of the invention is to provide a process for the production of polyurethane foams using carbon dioxide as blowing agent, wherein the pressure before expansion to normal pressure is minimal, i.e. wherein minimal demands can be made on the pressure-maintaining capacity of the foam-generating device (the perforated plates or screens according to the earlier proposals).
According to the earlier proposals mentioned above, bubble nucleation took place as the polyol/isocyanate mixture passed through the pressure-maintaining device. Bubble nucleation requires not only supersaturation of the mixture with carbon dioxide (i.e. the prevailing pressure is below the equilibrium solution pressure) but also the production of high shear rates. These high shear rates occur only in the marginal region of the apertures in the pressure-maintaining device. To generate a sufficiently large number of homogeneous bubble nuclei, it was therefore necessary either to choose very narrow apertures, for example with diameters of 50 xcexcm or less, or to impose high shear rates on the mixture several times intermittently by designing the pressure-maintaining device as a pack of several perforated plates. Thus a high-quality foam can be produced with a stack of 5 to 7 screens whose holes have diameters of 100 to 120 xcexcm. However, neither very small screen hole diameters nor a large number of stacked screens favour the concomitant use of fillers in the polyol/isocyanate mixture. With a number of stacked screens commensurate with the residence time of the mixture in the pressure-maintaining device, attempts to use perforated plates with hole diameters of about 200 xcexcm have regularly led to foam qualities which can no longer be utilized commercially under modem conditions.
It has been found that a sufficient number of bubble nuclei can be generated in polyol containing dissolved carbon dioxide if the polyol undergoes a sufficiently large pressure drop on passing through a nozzle. The number of bubble nuclei formed depends on the pressure drop on passage through the nozzle. A sufficient number of bubble nuclei for polyurethane foam production are generated if the pressure drop on passage through the nozzle corresponds to at least 5 times the equilibrium solution pressure of the dissolved carbon dioxide. The basic experiments have been carried out on polyol. The same applies to isocyanate and the polyol/isocyanate mixture.
By exploiting these observations, it is possible to disassociate the bubble nucleation from the expansion to normal pressure.
The present invention accordingly provides a process for the production of polyurethane block foam, wherein a polyurethane reactive mixture containing carbon dioxide is suddenly expanded from a pressure above the equilibrium solution pressure of the carbon dioxide to normal pressure, the liquid polyurethane reactive mixture foams with the release of dissolved carbon dioxide, and the foamed mixture is applied to a substrate and then cured to form block foam, characterized in that the carbon dioxide is initially completely dissolved in the reactive mixture, or in at least one of the components polyol and isocyanate, at a pressure substantially above the equilibrium solution pressure, the pressure is then reduced to a value close to the equilibrium solution pressure, falling in the meantime below the equilibrium solution pressure with the release of small amounts of carbon dioxide to form a bubble microdispersion, the component containing carbon dioxide is mixed with the other component, if appropriate, and the pressure is suddenly reduced to normal pressure without the released carbon dioxide being completely redissolved.
The magnitude of the pressure substantially above the equilibrium solution pressure before the first pressure reduction determines the number of bubble nuclei generated during the first pressure reduction. To produce a uniform small-cell block foam, it is necessary, depending on the proportion of dissolved CO2, to generate about 100,000 to 200,000 bubble nuclei per cm3 of liquid polyol/polyisocyanate mixture. This is generally the case when the pressure before the first pressure reduction is more than 5 times the equilibrium solution pressure of the dissolved carbon dioxide. The pressure before the first pressure reduction should preferably be between 8 and 15 times the equilibrium solution pressure of the dissolved carbon dioxide. Apart from the technological difficulties of pressure control, there is no upper limit to the pressure: the higher the pressure, the smaller the cells in the resulting block foam.
In principle, the first pressure reduction can be carried out after the polyol, isocyanate and carbon dioxide have been mixed, although the unit for mixing the polyol and isocyanate then has to be operated at high pressure.
Therefore, the carbon dioxide is preferably dissolved in the polyol component at high pressure and the polyol component containing carbon dioxide is introduced under reducing pressure into the mixing chamber in order to be mixed with the isocyanate. The pressure reduction can be effected by passage through a simple pressure-reducing valve, preferably in the form of an adjustable needle valve. Because of the high speed of passage through the valve, the pressure of the polyol/carbon dioxide mixture is momentarily reduced below the equilibrium solution pressure, bubble nuclei being generated by the high shear rates simultaneously produced.
The pressure in the mixing chamber is kept close to the equilibrium solution pressure of the carbon dioxide in the polyol/isocyanate mixture. As the polyol containing carbon dioxide is mixed with the isocyanate, the carbon dioxide concentration is reduced by the mixing in the ratio of the mixing components. Until it mixes with the isocyanate, the polyol containing carbon dioxide, injected into the mixing chamber, is at a pressure below the equilibrium solution pressure of carbon dioxide in isocyanate for a sufficient time to form stable bubble nuclei. In other words it is supersaturated with carbon dioxide. The typical period required to form stable bubble nuclei, i.e. bubble nuclei whose diameter is sufficient for the surface tension to prevent their tendency to redissolve, is 10xe2x88x924 to 10xe2x88x922 sec.
If the first pressure reduction takes place after the polyol and isocyanate have been mixed, care must be taken to ensure that the pressure is below the equilibrium solution pressure for a sufficient length of time. This can be achieved with appropriately designed Laval nozzles.
After the first expansion the pressure should be as close as possible to the equilibrium solution pressure of the dissolved carbon dioxide in the polyol/isocyanate mixture, i.e. not sufficiently above the equilibrium solution pressure to allow the bubble nuclei to redissolve and not sufficiently below the equilibrium solution pressure to allow the bubble nuclei to grow to bubble sizes that tend to coagulate.
Preferably, the equilibrium solution pressure should be exceeded by less than a pressure difference xcex94p of 0.5/t bar, where t is the time in seconds that elapses between the first and second pressure reductions. The time required between the first pressure reduction and the second pressure reduction is determined by the volume of apparatus (mixing chamber and pipelines) between the first and second pressure-reducing devices and the throughput of polyol/polyisocyanate mixture. This time is typically 0.5 to 6 seconds, preferably 2 to 4 seconds.
The amount by which the pressure falls below the equilibrium solution pressure between the first and second pressure reductions should preferably be less than 5%, particularly preferably less than 3% and very particularly preferably less than 2% of the equilibrium solution pressure. In this case the bubble nucleus diameters remain well below 100 xcexcm, even when an equilibrium is adjusted, so that there is substantially no coagulation of bubble nuclei.
Accordingly, for a polyol/isocyanate mixture which contains 4 parts by weight of carbon dioxide to 100 parts by weight of polyol and whose equilibrium solution pressure is about 5 bar, the pressure should typically be less than 0.1 bar above or below the equilibrium solution pressure. Variations arise from the different solubility of carbon dioxide in different qualities of raw materials and from specific apparatus conditions.
Accordingly, an essential feature of the present invention is sensitive adjustment and maintenance of the pressure of the polyol/isocyanate mixture between the first and second pressure reductions. This can be achieved in a number of ways: One possibility consists in maintaining the equilibrium solution pressure via the pressure-reducing device for the first pressure reduction. Another possibility is to adjust the mass fluxes of the components so that, for given apparatus conditions, the equilibrium solution pressure is assured between the first and second pressure reductions. The equilibrium solution pressure is preferably assured by using a pressure-reducing device of variable pressure-maintaining capacity for the second pressure reduction.
The invention also provides a pressure-maintaining device for the foaming of polyol/polyisocyanate mixtures containing carbon dioxide dissolved under pressure, and optionally containing fillers, said device comprising, at a housing outlet transverse to the direction of flow of the mixture, two parallel perforated plates whose separation is adjustable between 0.1 and 0.3 mm and which have the following features:
the perforated plates have a uniform perforation grid of constant pitch,
the two grids being offset by half a period relative to one another,
the hole density of the downstream perforated plate is 1.5 and 5 per cm2,
the free cross-sectional area of the upstream perforated plate is between 1.5 and 4%,
based on the total cross-section of the perforated plates, and
the free cross-sectional area of the downstream perforated plate is between 10 and
30%, based on the total cross-section of the perforated plates.
The pressure-maintaining device is preferably formed of two perforated plates, each of which has a trilateral or quadrilateral perforation grid.
Also, the pressure-maintaining device is preferably formed of an upstream perforated plate with a hexagonal perforation grid and a downstream perforated plate with a triangular grid, the hole density of the upstream perforated plate therefore being twice that of the downstream perforated plate. xe2x80x9cUpstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d relate to the separation gap between the perforated plates.
Particularly preferably, the through-holes in the downstream perforated plate are conically enlarged in the direction of flow, especially in such a way that the outlet of the perforated plate, i.e. the underside of the plate, is formed of sharp edges so that no voids are formed in the flow.